The present disclosure generally relates to metastable β-titanium alloys and methods of processing metastable β-titanium alloys. More specifically, certain embodiments of the present invention relate to binary metastable β-titanium alloys comprising greater than 10 weight percent molybdenum, and methods of processing such alloys by hot working and direct aging. Articles of manufacture made from the metastable β-titanium alloys disclosed herein are also provided.
Metastable beta-titanium (or “β-titanium”) alloys generally have a desirable combination of ductility and biocompatibility that makes them particularly well suited for use in certain biomedical implant applications requiring custom fitting or contouring by the surgeon in an operating room. For example, solution treated (or “β-annealed”) metastable β-titanium alloys that comprise a single-phase beta microstructure, such as binary β-titanium alloys comprising about 15 weight percent molybdenum (“Ti-15Mo”), have been successfully used in fracture fixation applications and have been found to have an ease of use approaching that of stainless steel commonly used in such applications. However, because the strength of solution treated Ti-15Mo alloys is relatively low, they are generally not well suited for use in applications requiring higher strength alloys, for example, hip joint prostheses. For example, conventional Ti-15Mo alloys that have been solution treated at a temperature near or above the β-transus temperature and subsequently cooled to room temperature without further aging, typically have an elongation of about 25 percent and a tensile strength of about 110 ksi. As used herein the terms “β-transus temperature,” or “β-transus,” refer to the minimum temperature above which equilibrium α-phase (or “alpha-phase”) does not exist in the titanium alloy. See e.g., ASM Materials Engineering Dictionary, J. R. Davis Ed., ASM International, Materials Park, Ohio (1992) at page 39, which is specifically incorporated by reference herein.
Although the tensile strength of a solution treated Ti-15Mo alloy can be increased by aging the alloy to precipitate α-phase (or alpha phase) within the β-phase microstructure, typically aging a solution treated Ti-15Mo alloy results in a dramatic decrease in the ductility of the alloy. For example, although not limiting herein, if a Ti-15Mo alloy is solution treated at about 1472° F. (800° C.), rapidly cooled, and subsequently aged at a temperature ranging from 887° F. (475° C.) to 1337° F. (725° C.), an ultimate tensile strength ranging from about 150 ksi to about 200 ksi can be achieved. However, after aging as described, the alloy can have a percent elongation around 11% (for the 150 ksi material) to around 5% (for the 200 ksi material). See John Disegi, “AO ASIF Wrought Titanium-15% Molybdenum Implant Material,” AO ASIF Materials Expert Group, 1st Ed., (October 2003), which is specifically incorporated by reference herein. In this condition, the range of applications for which the Ti-15Mo alloy is suited can be limited due to the relatively low ductility of the alloy.
Further, since metastable β-titanium alloys tend to deform by twinning, rather than by the formation and movement of dislocations, these alloys generally cannot be strengthened to any significant degree by cold working (i.e., work hardening) alone.
Accordingly, there is a need for metastable β-titanium alloys, such as binary β-titanium alloys comprising greater than 10 weight percent molybdenum, having both good tensile properties (e.g., good ductility, tensile and/or yield strength) and/or good fatigue properties. There is also a need for a method of processing such alloys to achieve both good tensile properties and good fatigue properties.